Method of conserving waterlogged materials

ABSTRACT

A method of preserving waterlogged materials and more specifically, preserving artifacts that have been submerged in water for long periods of time. A method of treating waterlogged materials that have been subjected to preservation techniques using polyethylene glycol as the preservative, and a method of restoring such materials from the deleterious effects of the glycol.

FIELD OF THE INVENTION

This invention deals with a method of preserving waterlogged materialsand more specifically, this invention deals with preserving artifactsthat have been submerged in water for long periods of time and further,this invention deals with waterlogged materials that has been subjectedto preservation techniques using polyethylene glycol as thepreservative, and a method of restoring such materials from thedeleterious effects of the glycol.

BACKGROUND OF THE INVENTION

The field of conservation of artifacts has drawn considerable interestin the past few years and such conservation practices have been utilizedto preserve archaeological materials. The chief concern in conservingthese materials is to bulk up the cell wall structure of the waterloggedmaterials by introducing compounds that replace water in the damagedstructure of the material. Allowing waterlogged wood to dry out withoutreplacing the water with a bulking agent results in warping andexcessive shrinkage, sometimes leading to total destruction of thewooden artifact.

Because the preserved wood needs to retain original coloration, havedimensional stability and integrity, one must be careful of thetechnique used to maintain these properties.

Prior art methods have used a variety of materials for preservation,namely, polyethylene glycol, acrylic resins and sugar.

For example, it is well-known in the field to use polyethylene glycolsas conservation materials, especially in wood substrates. However, thematerials so treated have a finite conservation time and one of theobjectives of the instant invention is to prolong theconservation/preservation time of such substrates. For example, one ofthe inventors herein had acquired some wooden tongue depressors made outof birch wood, that had undergone controlled degradation andconservation using a variety of molecular weights of polyethyleneglycol. It should be noted that polyethylene glycol has many physicalforms and that the higher molecular weights are preferred forconservation. Such higher molecular weights are normally solidmaterials. However, after nearly ten years of preservation, the tonguedepressors discussed supra showed signs of surface pooling and excessiveflexibility that suggest that the polyethylene glycol was in asemi-liquid form and that the polyethylene glycol used to preserve thewood had destabilized, which eventually leads to instability of thepreserved wood. In most cases, the coloration of the wood samples wastranslucent and unnatural in appearance. In some cases, the tonguedepressors maintained a wet appearance as well as a general heighteneddegree of flexibility that exceeded the range of flexibility noted inthe control samples.

Each of the above mentioned materials have their inherent benefits, butthey also have their inherent disadvantages, such as in the case of thepolyethylene glycol.

Because museums around the world have used a wide range of molecularweights of polyethylene glycol as bulking agents to conserve waterloggedand deteriorated wooden artifacts, and because polyethylene glycoltreated artifacts require controlled curation, new means of stabilizingthis bulking agent are necessary to ensure the long term survival oftreated artifacts.

Further, some of these methods require a post heating step and hightemperatures to complete the process and the wide variability intemperatures and high humidity can cause wood samples treated bypolyethylene glycol and/or sugar to weep, resulting in pooled liquids ontheir surfaces such as that suggested supra. Thus, highly controlledstorage of wood treated by these processes is essential for long termsurvival of the artifact.

Unfortunately, a great deal of wood recovered in the past from manyarchaeological sites was either ignored or discarded becauseconservation processes were either too costly or ineffective toguarantee results.

"Waterlogged" wood is employed herein as a reference to the wood whosewater content is above its fibre saturation point, that is, when all ofthe sites available for hydrogen bonding are no longer available.Typically, this wood has undergone biological decay with chemical andphysical changes inherent.

Under waterlogged conditions, water is in itself a conservation agentfor the wood. It acts as the bulking agent, helping the wood to retainits shape and some degree of integrity. When waterlogged wood is allowedto dry, it suffers severe dimensional changes due to the collapse of thecell cavities and shrinkage of the cell walls.

Thus, waterlogged wood conservation has as its main objective, theavoidance of shrinking and deformation of the wooden artifacts withoutcommensurate loss of aesthetics of the artifact.

Polyethylene glycol conservation of wood has been on-going since aboutthe 1940's when the potential for treating waterlogged wood was firstdiscovered. Polyethylene glycols are polymeric ethylene oxides that arecurrently commercially available from a number of manufacturers. Theyrange in physical appearance from liquids to semi-solid waxes, to hard,wax-like solids. Typically, polyethylene glycol conservation is carriedout in a batch process wherein the polyethylene glycol, having anaverage molecular weight of about 4000 is diluted with water and somesort of biocide. The wooden artifacts are immersed in this liquid, andheat is applied (generally less than 100° C.) to evaporate water andfacilitate penetration of the polyethylene glycol/biocide combination.It is known that the appropriate molecular weight of the polyethyleneglycol to use is a function of the degree of deterioration of the woodbeing treated. Lower molecular weights are advised for relatively soundwood and higher molecular weights for wood badly deteriorated.

Some methods require that the wood have its initial bulking waterdisplaced by using water soluble or water miscible solvents, followed bythe use of the polyethylene glycol dilute solutions.

THE INVENTION

The invention disclosed and claimed herein deals with a method ofconservation of wood substrates, especially wooden artifacts that havebeen immersed in water for long periods of time. Such a method comprises(I) impregnating a wooden substrate with a polyoxyethylene polymer or amixture of polyoxyethylene polymers having an average of at least twocarbinol groups per molecule and thereafter, (II) impregnating theproduct of (I) with sufficient crosslinker or a mixture of crosslinkersto crosslink a significant portion of the polyoxyethylene polymer ormixture of polyoxyethylene polymers of (I), and thereafter, (III)exposing the product of (II) to a catalyst or a mixture of catalysts fora time sufficient to initiate curing of the product of (II).

Another embodiment of this invention is one in which the woodensubstrates, which already contain polyoxyethylene polymers, can bepreserved. Thus, there is provided a method of preserving woodensubstrates that contain polyoxyethylene polymers, the method comprising:(I) impregnating the wooden substrate containing the polyoxyethylenepolymer with sufficient crosslinker or a mixture of crosslinkers tocrosslink a significant portion of the polyoxyethylene polymer ormixture of polyoxyethylene polymers, and thereafter, (II) exposing theproduct of (I) to a catalyst or a mixture of catalysts for a timesufficient to initiate curing of the product of (I).

There is yet another embodiment of the method disclosed herein whichmethod comprises (I) impregnating a wooden substrate with a curablesiloxane containing a siloxane polymer or a mixture of siloxane polymershaving an average of at least two silanol groups per molecule and asufficient amount of crosslinker or a mixture of crosslinkers tocrosslink a significant portion of the siloxane polymer or mixture ofsiloxane polymers, thereby displacing a portion or essentially all ofthe polyoxyethylene polymer from the wooden substrate and thereafter,(II) exposing the product of (I) to a catalyst or a mixture of catalystsfor a time sufficient to initiate curing of the product of (I).

Still another embodiment of this invention is a method comprising (I)impregnating a wooden substrate with a hydrolyzable silane or a mixtureof hydrolyzable silanes thereby displacing a portion or essentially allof the polyoxyethylene polymer from the wooden substrate and thereafter,(II) exposing the product of (I) to a catalyst or a mixture of catalystsfor a time sufficient to initiate curing of the product of (I).

The crosslinkers useful in this invention are generally selected fromthe group consisting of an alkoxysilane or a mixture of alkoxysilaneshaving the general formula R_(a) Si(OR')_(4-a) wherein R' is selectedfrom hydrogen, vinyl, or an alkyl group having from 1 to 6 carbon atoms.Also, such silanes can be functional silanes or a mixture of functionalsilanes having the general formula R_(a) Si (X)_(4-a) wherein R in eachformula is selected from the phenyl group, hydrogen, vinyl, or an alkylgroup having from 1 to 12 carbon atoms, a in each case has a value ofzero or 1 and X is selected from alkoxy, carboxy, oximo, enoloxy, amido,uriedo, carbamato, and amino.

Especially useful materials are methyltrimethoxysilane andisobutyltrimethoxysilane. Useful orthosilicates are ethylorthosilicateand tetraisopropylsilicate.

There is yet another method of preserving wooden substrates that containpolyoxyethylene polymer, the method comprising (I) impregnating a woodensubstrate with a cyclosiloxane or a mixture of cyclosiloxanes having anaverage of at least two silane hydrogens per molecule thereby displacingessentially all of the polyoxyethylene polymer from the woodensubstrate, and thereafter, (II) exposing the product of (I) to acatalyst or a mixture of catalysts for a time sufficient to initiatecuring of the product of (I).

There is yet another method of preserving wooden substrates that containpolyoxyethylene polymer, the method comprising (I) impregnating a woodensubstrate with a linear siloxane or a mixture of linear siloxanes havingan average of at least two silane hydrogens per molecule therebydisplacing essentially all of the polyoxyethylene polymer from thewooden substrate, and thereafter, (II) exposing the product of (I) to acatalyst or a mixture of catalysts for a time sufficient to initiatecuring of the product of (I).

EXAMPLE 1 Crosslinking Systems Using Polyethylene Glycol

Several combinations of crosslinking agents and catalysts were evaluatedfor the purposes of determining if polyethylene glycol could beconveniently crosslinked under the conditions used for the conservationof wooden artifacts.

In an attempt to duplicate the bulking of waterlogged wooden timbersfrom ships which were bulked using polyethylene glycol, a solution ofPEG 3350 (polyethylene glycol having a molecular weight of approximately3350 g/mole) was prepared by placing a jar of water into a warming ovenwhich was maintained at 158° F. Ten, ten gram portions of thepolyethylene glycol were slowly added to the water to give a solution ofapproximately 50 weight percent of polyethylene glycol.

Several hundred birch wood tongue depressors were placed in a four literstainless steel beaker and immersed in water. Mesh screen and a heavyweight were placed on top of the tongue depressors in solution toprevent them from floating during waterlogging processing. The wood,while still in solution was then placed on a warmer plate and heated toa temperature of 110° F. for eight hours per day for ten days. After thetenth day, the water was changed to fresh water and then the tonguedepressors were removed from the hot plate and stored in fresh water atroom temperature in a sealed jar. Samples of these woods were allowed toair dry at room temperature to determine if the structural integrity ofthe samples had been compromised. After twenty four hours, extensivewarpage and shrinkage of all of the samples suggested that the tonguedepressors has been sufficiently waterlogged for purposes of thetesting.

Twenty samples were selected and placed into a large jar of fresh water.This jar was then stored in a vented warming oven which was maintainedat a temperature of about 158° F. Ten weight percent increments of 3350powder were added to this water to form a solution, until 14 incrementshad been added, which resulted in a 58 weight percent PEG solution.

A single tongue depressor was removed from the PEG solution and surfacewiped with paper towel to remove free-flowing PEG solution from all ofits surfaces. This sample was dark brown in color. This tongue depressorwas sample 1. This sample was placed into a graduated glass cylindercontaining 50 milliliters of fresh methyltrimethoxysilane (MTM). A loosefitting cap was placed over the top of the cylinder and then the tonguedepressor in solution was placed into the vented warming oven. Thesample remained in the methyltrimethoxysilane in the oven for a periodof 24 hours and then it was removed from the oven and allowed to standin the solution at room temperature for 5 hours. The tongue depressorwas then removed from the MTM solution and lightly surface wiped withpaper towel.

The detailed apparatus and procedure were as follows. Each of theprepared samples when treated, was placed in an individual containmentchamber made of a one quart jar with a tight fitting lid. In an invertedposition, the lid of the unit formed a base, while the body of the jarformed a removable lid for the unit. An aluminum tray was used for thecatalyst and this try was placed in the center of the base and held inposition with a small piece of double sided tape. A small section ofexpanded aluminum mesh, approximately 1.75 inches square, was placedover the top of the catalyst tray. The edges of the mesh were then bentover, tightly securing the mesh to the tray. Small knife slits were madeon the top surfaces of the containment chamber lids as a means ofpreventing pressurization within the chambers during the treatment.

One and one half ounces of the catalyst of choice was placed into thetray and then the sample was placed into its respective containmentchambers resting on the mesh. In this position, the sample was placeddirectly over the top of the tray. Several small pieces of paper towelwere placed on top of the screen before the waterlogged tongue depressorwas placed in the unit to absorb free flowing PEG that otherwise mightcontaminate the catalyst. With the containment chamber lid firmly inplace, the containment chamber was placed into a vented warming oventhat was maintained at about 160° F. to create a catalyst vapor in thechamber. In the case of sample 1, dibutyltindilaurate was used. Allsamples, unless noted otherwise, were left in the catalyst vapordeposition for twenty four hours.

At the end of the treatment, each containment chamber was removed fromthe oven and carefully opened in a vented fume hood.

The sample 1 was light gray to brown in color. The edges of the tonguedepressor appeared to be somewhat translucent and generally, thesurfaces of the sample felt waxy and smooth to the touch. There was noshrinkage, even after several weeks of exposure to air. The sample hadnot warped or changed structurally.

EXAMPLE 2

Two tongue depressors that had been initially treated to waterlog themwere treated as in the method used in example 1 except that the catalystwas tetraisopropyltitanate. These samples, designated 2 and 3 hadessentially the same appearance as sample 1.

EXAMPLE 3

A tongue depressor that had been initially treated to water log it wastreated as in the method used in example 1, except that the catalyst wastin octoate. This sample was sample 4 and it had essentially the sameappearance as the previous samples.

EXAMPLE 4

Another treated tongue depressor was treated with MTM as above, and theonly exception was that the sample was treated for six hours instead of24 hours. This sample was sample 5 and it had essentially the sameappearance as the previous samples.

EXAMPLE 5

A treated tongue depressor was treated with MTM as above, and the onlyexception was that the sample was treated using MTM in which 3 weightpercent dibutyltindilaurate had been added.

The results of this testing can be found on TABLE I below.

                  TABLE I                                                         ______________________________________                                        SAMPLE      RESULTS                                                           ______________________________________                                        1           slightly browner in color than non-treated                                    no warpage, slight swelling                                       2           essentially same color as non-treated                                         no warpage, slight swelling                                       3           good color, slight swelling                                       4           gray-brown color, slight swelling                                 5           slightly gray-brown                                                           slight swelling                                                   ______________________________________                                    

EXAMPLE 6

Using gloves and a hot air gun, a water logged tongue depressor wasslowly warmed. The side surface of a one pint tin can was also warmedwith the air gun and while both the tongue depressor and can were stillhot, the tongue depressor was wrapped around the side of the can andheld in place until the can and tongue depressor had returned to roomtemperature, about 20 minutes. Rubber bands were then stretched aroundthe tongue depressor and tin can and positioned so that they held thetongue depressor in its wrapped position against the side of the can.The tongue depressor wrapped can was then placed in a 5 quart can and aweight was placed on top of the one pint can to prevent it fromfloating. Approximately 1 liter of MTM was poured into the 5 quart canso that the tongue depressor, which was wrapped on the side of the smallcan was submerged in the MTM. Once a loose fitting lid was positioned ontop of the larger outer vessel, the unit was placed into a ventedwarming oven set at about 160° F., and left in the treatment mode forabout 4 hours. The entire unit was then removed from the oven, and oncemoved to a vented fume hood, the small can and tongue depressor wereremoved from the warm MTM solution.

Still wrapped on the small can, the tongue depressor was then placedinto a large containment chamber resting on expanded aluminum mesh, overa catalyst tray containing three ounces of dibutyltindilaurate. With thelid of the containment chamber in position, the unit was placed into avented warming oven for approximately eighteen hours. The sample in itscontainment chamber was then removed from the oven and opened in avented fume hood. The rubber bands were removed and the semi-circleshaped tongue depressor slid easily from the side of the tin can.

The tongue depressor curved figure was traced on a piece of paper inorder to track any unlocking of the configuration. Over a period oftime, the tongue depressor configuration has changed very little.

EXAMPLE 7

Six waterlogged tongue depressors were randomly selected and immediatelyplace into one liter of fresh acetone and placed in a freezer mountedvacuum chamber (FMVC) and processed for 24 hours under vacuum to removethe water in the wood. After the water/acetone displacement, the sampleswere then placed into a container with 1 liter of hydroxy end-blockedpolydimethylsiloxane having a molecular weight of about 7600 g/mole andcontaining about 3 weight percent MTM. The samples were weighted down inthe solution. The container with the samples was placed into a freezerfor FMVC treatment. A vacuum was applied to the samples in solution for24 hours. Afterwards, the container containing the tongue depressors wasremoved from the freezer and the tongue depressors were removed from thepolydimethylsiloxane and lightly wiped with paper towel to removefree-flowing siloxane from the surfaces of the tongue depressors.

The tongue depressors were then placed into a one quart jar as a closedcontainment chamber system for catalyst vapor deposition. In the centerof the base of the containment chamber was placed the aluminum tray andit was fastened in place using a small piece of double sided tape. Smallpieces of paper towel were piled on the screen to prevent anyfree-flowing siloxane from dripping into the catalyst tray.

Twenty grams of dibutyltindilaurate were placed in the catalyst tray andthe chamber was placed in position and the entire unit was placed into avented warming oven maintained at about 160° F. The samples were leftfor sixty hours.

Upon removal from the system and upon evaluation of the samples, it wasnoted that they were slightly rubbery in texture and two of the six hadshrunk slightly.

Microscopic analysis of thin sections of a tongue depressor indicatedthat generally, all of the cells of the wood contained curedpolysiloxane.

What is claimed is:
 1. A method of conservation of wood substrates, saidmethod comprising:(I) impregnating a wooden substrate with apolyoxyethylene polymer or a mixture of polyoxyethylene polymers havingan average of at least two carbinol groups per molecule, and thereafter,(II) impregnating the product of (I) with sufficient crosslinker or amixture of crosslinkers to crosslink a significant portion of thepolyoxyethylene polymer or mixture of polyoxyethylene polymers of (I),whereas the significant portion is an amount to maintain dimensionalstability of the wooden substrate, and thereafter, (III) exposing theproduct of (II) to a catalyst or a mixture of catalysts for a timesufficient to initiate curing of the product of (II), wherein thecrosslinkers area functional silane or a mixture of functional silaneshaving the general formula:

    R.sub.a Si (X).sub.4-a

wherein R in (i) and (ii) is selected from the phenyl group, hydrogen,vinyl, or an alkyl group having from 1 to 12 carbon atoms, a in eachcase has a value of zero or 1 and X is selected from alkoxy, carboxy,oximo, enoloxy, amido, uriedo, carbamato, and amino.
 2. A method asclaimed in claim 1 wherein the impregnation of (I) is assisted bynegative pressure.
 3. A method as claimed in claim 1 wherein theimpregnation of (II) is assisted by negative pressure.
 4. A method asclaimed in claim 1 wherein both the impregnation of (I) and (II) areassisted by negative pressure.
 5. A method as claimed in claim 1 whereinthe impregnation of (I) is assisted by positive pressure.
 6. A method asclaimed in claim 1 wherein the impregnation of (II) is assisted bypositive pressure.
 7. A method as claimed in claim 1 wherein both theimpregnation of (I) and (II) are assisted by positive pressure.
 8. Aproduct when prepared by the method of claim
 1. 9. A method ofconservation of wooden substrates that contain polyoxyethylene polymersor a mixture of polyoxyethylene polymers, the method comprising:(I)impregnating the wooden substrate containing the polyoxyethylene polymerwith sufficient crosslinker or a mixture of crosslinkers to crosslink asignificant portion of the polyoxyethylene polymer or mixture ofpolyoxyethylene polymers, whereas the significant portion is an amountto maintain dimensional stability of the wooden substrate, andthereafter, (II) exposing the product of (I) to a catalyst or a mixtureof catalysts for a time sufficient to initiate curing of the product of(I), wherein the crosslinkers area functional silane or a mixture offunctional silanes having the general formula:

    R.sub.a Si (X).sub.4-a

wherein R in (i) and (ii) is selected from the phenyl group, hydrogen,vinyl, or an alkyl group having from 1 to 12 carbon atoms, a in eachcase has a value of zero or 1 and X is selected from alkoxy, carboxy,oximo, enoloxy, amido, uriedo, carbamato, and amino.
 10. A method asclaimed in claim 9 wherein the alkoxysilane is isobutyltrimethoxysilane.11. A method as claimed in claim 9 wherein the crosslinker istetraethylorthosilicate.
 12. A method as claimed in claim 11, whereinthere is additionally present an alkoxysilane or a mixture ofalkoxysilanes having the general formula:

    R.sub.a Si(OR').sub.4-a

wherein R is selected from the phenyl group, hydrogen, vinyl, or analkyl group having from 1 to 12 carbon atoms and R' is selected fromhydrogen, vinyl, or an alkyl group having from 1 to 6 carbon atoms and ahas a value of zero or
 1. 13. A method as claimed in claim 9 wherein theimpregnation of (I) is assisted by negative pressure.
 14. A method asclaimed in claim 9 wherein the impregnation of (I) is assisted bypositive pressure.
 15. A product when prepared by the method of claim 9.16. A method of conservation of wood substrates, said methodcomprising:(I) impregnating a wooden substrate with a mixture of:(i) atleast one polyoxyethylene polymer having an average of at least twocarbinol groups per molecule with (ii) sufficient crosslinker or amixture of crosslinkers to crosslink a significant portion of thepolyoxyethylene polymer, whereas the significant portion is an amount tomaintain dimensional stability of the wooden substrate, and thereafter,(II) exposing the wooden substrate to a catalyst or a mixture ofcatalysts for a time sufficient to initiate curing of the product of(I).